In this post we will be describing how to diagnose MOST bus failure in any BMW, AUDI, PORSCHE, ROLLS ROYCE, RANGE ROVER HSE, BENTLEY with MOST fiber optic bus. every car has different configuration, but principle is still the same!
So in 2000 they introduced newer multimedia bus, called MOST, IT IS VERY NICE AND FAST NETWORK, SO THERE IS ENOUGH SPEED TO CONTROL EVERY MULTIMEDIA DEVICE AND THEY ELIMINATED LOTS OF COPPER WIRES IN THE CAR. NOW SOUND IS FULLY DIGITAL AND CLEANEST (BC NO SIGNAL LOSS ON THE WIRES), SO IT IS A GOOD THING, BUT ITS MORE COMPLEX AND NOT VERY RELIABLE.
So to start you need to understand that all MOST (multimedia) modules in the car work in the closed loop, which means if any device in the loop fails-none will be working.MOST bus primary signal (optical red light signal) is generated inside of the main computer, which is your main screen CID (control display or CID in E65 car only, other BMW have main computer incorporated into head unit or CCC/CIC/NBT etc), then it travels through all MOST network devices (ask unit, amp, navi, fm tuner, phone etc. depending on how many options you have) and comes back into main head unit. Basically all units connected in the loop- one unit fails=broken network! Once you understand this principle, you can easily find broken unit or network interruption, don’t forget dealer will charge up to 500-1000$++ for such a serious job! You don’t believe me? go and try, then you will understand how priceless with post is.
Your E65 E66 or Rolls Royce phantom ( yes don’t be surprised it is the same exact car as your 760li btw) car has MOST bus failure if:
-cannot hear any sounds or music
-cannot see tracks playing on the display
-fm stations not available
-navi isn’t functioning
For other BMW like e60/e90/e70/e89/f01 and others MOST BUS FAILURE symptoms are little different because primary head unit has 4-5 modules (SVS, ask unit, MOST GATEWAY, NAVI, Radio tuner) combined and therefore everything will work except for those secondary modules:
-amplifier no sound and you see mute icon on the radio or screen
-TCUM or telephone module
-SAT NAV receiver
Here is the MOST DEVICES order diagram , but this is only valid for basic order and only for pre 2006 cars, you might have way more devices in the network, so you have to figure it out yourself by looping eac modules and draw a diagram of the modules.
Here is a quick video showing how to find a bad unit in real time:
FOR 2006-2008 E65 E66 cars basic order will look like this:
MMIGT(screen)>>CDC>>AVT(if you have it)>>TEL>>SAT(hd RADIO)>>L7>>SVS>>NAVI>>ASK>>OOPS(GLOVE BOX MOST LOOP)>>MMIGT
FOR E60 E90 E63 E89 E90 E92 E93 and so on:
HEAD UNIT CCC/CIC/NBT>>L7AMP>>SAT>>TCU>>MULF>>HEAD UNIT
On these cars in most cases i see sat and amplifier unit failure very often, telephone module goes bad too but not that often.CCC navi units have a high rate failure, but the good thing once you replace it, you replace 5 modules at once, so its more reliable and much easier to find a faulty unit because you only have 2 or 3 modules to diagnose and all of them located in the trunk. We also offer CCC or CIC repair service
So to find out the faulty unit you either need to see a dealer (but even dealer makes mistakes), but here is an easy solution if you don’t have anything, and if you do it right it always works:
You need basically to find out where the most signal gets lost, after what device you don’t see the red light. In other words you will be checking the red light after every device in the network, and the one which is not sending the red light signal is your FAULTY MOST UNIT. OOPS is the last diagnostic connection located in the glove box and it has lollipop attached and it has to be looped there, some people think it is a spare jumper, but its not, without this jumper you will interrupt the network!
HOW TO BYPASS NON WORKING MODULE??? EASY! GET THIS P/N FROM ANY BMW DEALER 61136917541, YOU CAN GET IT ONLINE FROM ECS TUNNING ECS TUNNING FIBRE OPTIC JUMPER. YOU HAVE TO REMOVE INNER PART WITH SOME FLAT SCREWDRIVER OR A KNIFE, YOU CAN ALSO USE ZIP TIE IF YOU PLAN TO KEEP THE JUMPER BYPASSED, IRT WON’T STAY WITHOUT ZIP TIE.
most common MOST devices failure (descending from most common to less):
-SAT SIRIUS unit always goes bad, we will offer repair of this unit soon
–TCU TELEPHONE BLUETOOTH MODULE RECENTLY WE SEE TONS OF THEM FAILING!
-Logic7 amp AMP
-ask cd unit ASK
-navigation NAVI
-fm tuner
-CD itself main screen CID
-cd changer CDC
-SVS (gets flooded with hydraulic fluid from trunk opener cylinder, if you see sticky fluid on the NAVI, SVS- replace asap)
other modules. depending what you have in your car
Always check if you have red light in the glove box, disconnect that jumper and check for the light, if you found and bypassed bad unit correctly you should see the red light in the glovebox and then you can install the jumper and everything should start working. I know it sound complicated but once you understood how it works it is kind of very easy.
BUT PLEASE DO NOT BLINDLY BYPASS EVERY MODULE, LIKE LOTS OF PEOPLE DO, YOU MIGHT FIND THE PROBLEM, BUT ITS NOT EFFICIENT AND SOMETIMES YOU CANNOT DIAGNOSE IT RIGHT. ALSO MAKE SURE EVERY MODULE HAS 12v POWER. BASICALLY EVERY MODULE NEEDS FIBER OPTIC SIGNAL AND 12V POWER ATTACHED TO FUNCTION.TO WAKE UP MOST NETWORK IT IS ENOUGH TO OPEN/CLOSE THE DOOR, YOU DON’T HAVE TO START THE CAR, THE THING IS MOST GATEWAY WILL GO TO SLEEP AFTER 10 MINUTES IF NETWORK IS BROKEN AND TO WAKE IT UP YOU NEED TO OPEN/CLOSE ANY DOOR OR TURN ON THE RADIO. 
THAT’S about it, IF YOU FOUND THE BAD MODULE YOU JUST SAVED 300-400$ AND TIME. REMEMBER LOTS BMW SHOPS STILL CANNOT DIAGNOSE IT RIGHT AND ALWAYS POINT TO ASK UNIT OR AMPLIFIER, “BECAUSE THERE IS NO SOUND”, WHICH IS NOT ALWAYS TRUE, I WOULD SAY IN 50% ITS NOT CORRECT EVEN FROM OFFICIAL DEALERS. WHY THEY MADE IT SO COMPLICATED AND NOT RELIABLE?? WELL THIS A FUTURE OF CARS AND TECHNOLOGY, YOU ARE THE ONE WHO ASKED TO HAVE ALL THESE TOYS IN THE CAR, SO YOU EITHER LEARN IT OR HAVE YOUR WALLET READY TO GET SKINNIER, OR BUY HONDA/TOYOTA….
I recently purchased a VERY low miles 2016 F13 M6. Initially all worked great, but the car sat for a full month after I had major surgery. Barely survived, so am quite grateful! A friend and I took the car out for a gentle ride this past Saturday, no gongs, no sounds from the stereo etc. Bluetooth connects to devices, and menus show correctly. Navi works. Phone connects. CD plays, AM/FM shows accurate Station ID. time, track info etc. XM satellite stations register as active, and show every updated song title in real time. There is no water intrusion or sign of corrosion on any of the connections pins, nor under the cover of the amp. When I check the amp, it shows red and yellow in the amp module, but I have zero idea where else to look. The black fiber connects are not easily removed. If the amp is bad, easy, I send it in, and have you rebuild it. That comes out no problem, but I do see a constant red, and yellow LED inside when the car is powered up, and radio/infotainment system is powered up.
Does it say “no warning tones” message on the screen??? Will it say it, if you disconnect fiber optic plug from the amp?
Your problem is most likely the amp
Yeah surgeries suck, sometimes they resolve nothing…
Hello! Hope you can help me.. I have an 08 E93 M3 and I can listen to music but it only plays from the front and subs.. I have no sound out of the rear speakers.. also no PDC beeps when reversing. Could this be amp/ PdC module? If amp, how much to repair individual amp?
Thanks!
amp problem
Good morning,
I have a 2008 BMW 750i, and the problem that I’m experiencing is that the volume goes up at random times; I lower it manually and will stay there for the duration of the trip.
What could be causing this, I have also developed a low level battery drain (.2 amps) could these 2 issues be related?
Thanks for sharing your expertise.
strange, may be speed volume feature is on??
07 750li with bad speakers I have no sounds coming out of front doors and on rear deck. The speakers that do work are the rear doors and the center console speaker bt they sound bad. My navigation, Bluetooth, radio etc all work. When phone is connected though I can’t talk on it I can only hear person I’m talking to. I have no self belt sound door chimes Navi voice or key detection sounds. I do though have parking sounds, only for the back though no beeping for front I believe. Is it my ask or my amp I checked speakers door module and voice control module already.
both ask unit and amp must be redone
BMW 2016 428i convertible with Harmon Kardon Headunit and speaker. Everything was working fine until a few months back. All of the functions work on the radio but my sound cuts out intermittently and sometimes it is out for long periods. It suddenly comes on while driving and then goes out all of a sudden. I used Bimmerlink to diagnose the problem and I saw two errors 1) Amplifier – undervoltage detected 2) Headunit – MOST configuration incomplete.
I checked the fiber optic connection coming into the HK amp and that shows red light when ignition on. I do see a black box Bluetooth Telematic Control Unit in the trunk. Does this also get the fiberoptic connection? Where would the Satellite module be?
Appreciate if anyone can guide me with the sound issue
in your car you only have amplifier on MOST bus, since all other modules are built into head unit. YOUR PROBLEM IS ACTUAL AMPLIFIER, they fail quite often and especially convertibles due to water intrusion. contact us and we can help if it wasnt fully submerged.
its amp issue and we can fix it, you don’t have any other units in the trunk, only amp
Hello,
I have a 730d e65 2004. I have no song from radio. I can’t scan ou select radio station. I just have song from cd (ask I guess) but just from the back. No doors songs or belts songs. And I have red line in the glove box.
Many thanks.
Alex
ask unit needs repair
My sound recently went out, no door chimes, seat belt or radio sounds and I’ve spent a few days in this. I have a 2007 E92 335i coupe, CCC Head Unit, Logic 7 Amp, and TCU Bluetooth. The Logic 7 amp has a solid red service LED instead of a blinking orange unit. My guess is the head unit is bad. I followed your guide. My jumper (Last fiber optic cable before returning to the head unit) is under the steering wheel. I unattached it and there is a red blinking light. I then removed the Fiber optic from the TCU, and my last fiber optic lost the red light as expected but the TCU Fiber Optic cable was blinking red. I looped the TCU cable and the last Fiber optic (jumper) blinked red so I take it those two modules are good. I plugged them all back in, and then unplugged the Fiber optic from the Logic7, and it was blinking red. The only thing I can’t test is the head unit. Is it safe to say the head unit may be the problem? Help would be greatly appreciated and if a repair is needed, please let me know which device, the cost and how I can get it to you. Thank you.
its not head unit, but something else. amp has yellow LED blinking?
I have an e90, my head unit lights up briefly and shows date and time and then goes black. When I go to the amp etc the it is a flashing red light I stead of steady red light does this indicate a bad head unit? And can you repair the head unit?
CCC most likely at fault
Hello, it appears that each module is receiving the signal in my E71, but I have no base in the car anymore. All speakers work, but there is zero bass in the car. The amp is a Top Hi-Fi system. I dont think both subs will go out at once?
they could
2007 550i e62…replaced ccc (used) but same #’s…it may not be logic 7… my ccc did not have coax going in top left side (in rear) the used one did….any way .all I had was some radio and cd in glove now I have everything working except radio… can hear the speakers when turned up? do I and can you code it in if needed? Thanks….Mike in NC
coding is a must